我试图在一种情况下利用分区的优势：我有表“事件”，该表按字段“ dt_pk”按列表划分，该字段是表“日期”的外键。
-- Schema
drop schema if exists test cascade;
create schema test;
-- Tables
create table if not exists test.dates (
id bigint primary key,
dt date not null
);
create sequence test.seq_events_id;
create table if not exists test.events
(
id bigint not null,
dt_pk bigint not null,
content_int bigint,
foreign key (dt_pk) references test.dates(id) on delete cascade,
primary key (dt_pk, id)
)
partition by list (dt_pk);
-- Partitions
create table test.events_1 partition of test.events for values in (1);
create table test.events_2 partition of test.events for values in (2);
create table test.events_3 partition of test.events for values in (3);
-- Fill tables
insert into test.dates (id, dt)
select id, dt
from (
select 1 id, '2020-01-01'::date as dt
union all
select 2 id, '2020-01-02'::date as dt
union all
select 3 id, '2020-01-03'::date as dt
) t;
do $$
declare
dts record;
begin
for dts in (
select id
from test.dates
) loop
for k in 1..10000 loop
insert into test.events (id, dt_pk, content_int)
values (nextval('test.seq_events_id'), dts.id, random_between(1, 1000000));
end loop;
commit;
end loop;
end;
$$;
vacuum analyze test.dates, test.events;
我想像这样运行选择：
select *
from test.events e
join test.dates d on e.dt_pk = d.id
where d.dt between '2020-01-02'::date and '2020-01-03'::date;
但是在这种情况下，分区修剪不起作用。很明显，分区键没有常量。但是从documentation中，我知道执行时会进行分区修剪，它适用于从子查询获得的值：
分区修剪不仅可以在计划给定查询，也可以在执行期间。这很有用当子句包含表达式时，允许修剪更多分区其值在查询计划时未知，例如，在PREPARE语句中定义的参数，使用从子查询，或在内部使用参数化值嵌套循环联接。
所以我这样重写查询，并希望在修剪中进行分区：
select *
from test.events e
where e.dt_pk in (
select d.id
from test.dates d
where d.dt between '2020-01-02'::date and '2020-01-03'::date
);
但是此选择的explain说：
Hash Join (cost=1.07..833.07 rows=20000 width=24) (actual time=3.581..15.989 rows=20000 loops=1)
Hash Cond: (e.dt_pk = d.id)
-> Append (cost=0.00..642.00 rows=30000 width=24) (actual time=0.005..6.361 rows=30000 loops=1)
-> Seq Scan on events_1 e (cost=0.00..164.00 rows=10000 width=24) (actual time=0.005..1.104 rows=10000 loops=1)
-> Seq Scan on events_2 e_1 (cost=0.00..164.00 rows=10000 width=24) (actual time=0.005..1.127 rows=10000 loops=1)
-> Seq Scan on events_3 e_2 (cost=0.00..164.00 rows=10000 width=24) (actual time=0.008..1.097 rows=10000 loops=1)
-> Hash (cost=1.04..1.04 rows=2 width=8) (actual time=0.006..0.006 rows=2 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 9kB
-> Seq Scan on dates d (cost=0.00..1.04 rows=2 width=8) (actual time=0.004..0.004 rows=2 loops=1)
Filter: ((dt >= '2020-01-02'::date) AND (dt <= '2020-01-03'::date))
Rows Removed by Filter: 1
Planning Time: 0.206 ms
Execution Time: 17.237 ms
因此，我们读取了所有分区。我什至试图让计划者使用嵌套循环连接，因为我在文档中读到了“ 嵌套循环连接内侧的参数化值”，但是它没有用：
set enable_hashjoin to off;
set enable_mergejoin to off;
再次：
Nested Loop (cost=0.00..1443.05 rows=20000 width=24) (actual time=9.160..25.252 rows=20000 loops=1)
Join Filter: (e.dt_pk = d.id)
Rows Removed by Join Filter: 30000
-> Append (cost=0.00..642.00 rows=30000 width=24) (actual time=0.008..6.280 rows=30000 loops=1)
-> Seq Scan on events_1 e (cost=0.00..164.00 rows=10000 width=24) (actual time=0.008..1.105 rows=10000 loops=1)
-> Seq Scan on events_2 e_1 (cost=0.00..164.00 rows=10000 width=24) (actual time=0.008..1.047 rows=10000 loops=1)
-> Seq Scan on events_3 e_2 (cost=0.00..164.00 rows=10000 width=24) (actual time=0.007..1.082 rows=10000 loops=1)
-> Materialize (cost=0.00..1.05 rows=2 width=8) (actual time=0.000..0.000 rows=2 loops=30000)
-> Seq Scan on dates d (cost=0.00..1.04 rows=2 width=8) (actual time=0.004..0.004 rows=2 loops=1)
Filter: ((dt >= '2020-01-02'::date) AND (dt <= '2020-01-03'::date))
Rows Removed by Filter: 1
Planning Time: 0.202 ms
Execution Time: 26.516 ms
然后，我注意到在“执行时分区修剪”的每个示例中，我仅看到=条件，而不是in。它确实可以这样工作：
explain (analyze) select * from test.events e where e.dt_pk = (select id from test.dates where id = 2);
Append (cost=1.04..718.04 rows=30000 width=24) (actual time=0.014..3.018 rows=10000 loops=1)
InitPlan 1 (returns $0)
-> Seq Scan on dates (cost=0.00..1.04 rows=1 width=8) (actual time=0.007..0.008 rows=1 loops=1)
Filter: (id = 2)
Rows Removed by Filter: 2
-> Seq Scan on events_1 e (cost=0.00..189.00 rows=10000 width=24) (never executed)
Filter: (dt_pk = $0)
-> Seq Scan on events_2 e_1 (cost=0.00..189.00 rows=10000 width=24) (actual time=0.004..2.009 rows=10000 loops=1)
Filter: (dt_pk = $0)
-> Seq Scan on events_3 e_2 (cost=0.00..189.00 rows=10000 width=24) (never executed)
Filter: (dt_pk = $0)
Planning Time: 0.135 ms
Execution Time: 3.639 ms
这是我的最后一个问题：执行时分区修剪是否仅适用于子查询返回一个项目，或者有一种方法可以利用子查询返回列表来获得分区修剪的优势？
并且为什么它不能与嵌套循环连接一起使用，我在语言上理解不对吗：
这包括子查询的值和执行时的值参数，例如来自参数化嵌套循环联接的参数。
或“ 参数化嵌套循环连接”与常规嵌套循环连接有所不同吗？
嵌套循环联接中没有分区修剪，因为分区表位于外侧，始终会被完全扫描。使用来自外部的连接键作为参数扫描内部（因此进行参数化扫描），因此，如果分区表位于嵌套循环连接的内部，则可能会发生分区修剪。
[如果您不关心细节，更关心它是否可以正常工作，并且还没有尝试过：您可以将查询重写为类似的内容